Thank you both. I think I understand things a bit better than before. I think the idea of using a gross zero crossing aproximator to drive the PLL into the neighborhood of the incoming frequency and then letting the loop filter / NCO take over sounds like a very workable idea. Tim, as I indicated, I have been planning this as a future sub project for a while now. I have played around with the PFD concept multiplying a reference and "line" (sine and cosine waves at various magnitudes, frequencies and phase offsets) and filtering the result in mathcad to get an intuitive feel for the operation and effects. From these experiments, I believe I understand what you are saying about the phase error I and Q rotating (like a vector spinning in time) and how depending on the phase relationship of the signals one will be high, the other will be low. The big part that I haven't addressed in time is the part about driving the NCO. I realize that the output of the loop filter does this, but I suspect part of the trick will be determining the gain or attenuation factor to the NCO, but that can wait till another day.
Re: Software PLL (SPLL)
Started by ●February 7, 2006